Page 45 - Carbonate Sedimentology and Sequence Stratigraphy

36 WOLFGANG SCHLAGER

DESCRIPTION ORIGIN OF GRAINS
Skeletal grains: calcareous hard parts of organisms or Secreted by organisms in biotically-controlled fashion; specific shapes and textures
fragments of them. provide important clues to the organism and the environent of formation.
Peloids: micritic, subrounded grains, usually without Can originate (1) as fecal pellets of mud-digesting organisms; (2) by micritization
internal structure; soft or hard during deposition (alteration by boring microbes) of skeletal grains or ooids; (3) by precipitation of
micrite in biotically-induced fashion.
Aggregate grains: lumps of several round grains, Delicate balance between deposition, erosion and cementation at the sea floor.
agglutinated by sparitic or micritic cement or micrite Sand-size grains are first deposited, then weakly cemented in a patchy manner, and
laminae. subsequently ripped apart by renewed currents such that only the best-cemented
groups stay together.
Oncoids: irregularly shaped grains consisting of crudely Micrite layers are precipitated in bio-induced fashion around a nucleus and this lump
concentric to strongly asymmetric micritic laminae wrapped is occasionally rolled around to become coated on all sides; precipitation mainly by
around a nucleus. Oncoids may be hard or soft upon bacteria or cyanobacteria.
deposition.
Ooids: Smooth, spherical grains consisting of a nucleus Smooth surfaces and near-spherical shape indicate formation in an environment
and concentric laminae. Laminae may be micritic or where physical abrasion in turbulent water and growth alternate. Ooids of different
sparitic, sometime displaying radial structure super- mineralogies and textures have been produced by abiotic precipitation in the
imposed on the concentric fabric. Most ooids are sand size, laboratory (Davies & Ferguson, 1978) but bio-induced precipitation cannot be
coarser ooids are often distinguished as "pisoids". excluded for natural envionments.
Cortoids: grains with the principal structure of oncoids or Formation analogous to oncoids or ooids except that the grain reaches limiting size
ooids but consisting of a large nucleus covered by very few already after few laminae because of the large nucleus. Some cortoids form by
concentric micrite layers. micritization of the outermost parts of skeletal grains or ooids.
Lithoclasts: fragments of limestone or dolomite; as Reworking of lithified layers or rocks by marine or terrestrial erosion or biotic activity.
carbonate sediment may lithify in a few years within the
depositional environment, lithoclasts may be geologically
coeval and derived from the same environment. These
clasts are sometimes called "intraclasts" and distinguished
from externally derived, stratigraphically older "exoclasts".

Fig. 2.33.— Carbonate grains. Compiled from Flügel (1982, p. 124); Tucker and Wright (1990, p. 1-13); Flügel, 2004, p. 100-107.

original components not bound together during original components were bound together original components not
deposition during deposition bound together during

generally smaller grains (arenite and silt size) more than 10 % larger
grains (rudite size)
contains mud (micrite matrix) lacks mud Organisms act Organisms act Organisms act contains lacks mud
(sparite matrix) as sediment as sediment as frame mud (micrite (sparite
bafflers (e.g. binders (e.g. builders (e.g. matrix) matrix)
less than more than dendroid algal mats) intergrown reef
10% grains 10% grains corals) corals)

mud- grain-
mud-supported grain-supported Boundstone
supported supported

Mudstone Wackestone Packstone Grainstone Bafflestone Bindstone Framestone Floatstone Rudstone

Fig. 2.34.— Classification of carbonate rocks based on Dunham (1962) and Embry and Klovan (1971).